1. Filed of the Invention
The present invention relates to a fuel electrode for a solid polymer electrolyte fuel cell, a solid polymer electrolyte fuel cell and relates to a method for controlling a solid polymer electrolyte fuel cell.
2. Description of the Related Art
Among a number of types of fuel cells such as phosphoric acid fuel cells, molten-carbonate fuel cells, solid electrolyte fuel cells and solid polymer electrolyte fuel cells, the solid polymer electrolyte fuel cells are considered to be promising due to its compact configuration and high-output operations at low temperatures.
In the solid polymer electrolyte fuel cells, hydrogen as fuel is consumed at a fuel electrode, which is an anode. As a result, hydrogen ions and electrons are produced according to an ionization reaction expressed by the following reaction formula (1).2H2→4H++4e  (1)
In addition, oxygen, hydrogen ions and electrons are consumed at an oxidant electrode, which is a cathode, water is produced due to an ionization reaction expressed by the following reaction formula (2).O2+4H++4e−→2H2O  (2)
Then, among the products produced from the reaction expressed by the reaction formula (1), hydrogen ions and electrons move from the fuel electrode to the oxidant electrode via an electrolyte comprising a polymer ion exchanging membrane interposed between the fuel electrode and the oxidant electrode for consumption by the reaction expressed by the formula (2). Electrons among the products move from the fuel electrode to the oxidant electrode via an external circuit connecting the fuel electrode with the oxidant electrode for consumption by the reaction expressed by the formula (2).
As this occurs, since the electrolyte of this fuel cell is prepared in an acid environment, the equilibrium potential EH2 is expressed by the following Formula 1 from Nernst equation.
                              E                      w            1                          =                              RT                          2              ⁢              F                                ⁢          ln          ⁢                                                    a                2                            ⁢                              H                T                                                    aH              2                                                          [                  Formula          ⁢                                          ⁢          1                ]            
(In Formula 1, R denotes a gas constant, T denotes Kelvin temperature, F denotes Faraday constant, and a denotes activity.)
In addition, the equilibrium potential Eo2 of the oxidant electrode of this fuel cell is expressed by the following Formula 2.
                              E          01                =                              E            02                    +                                    RT                              2                ⁢                F                                      ⁢            ln            ⁢                                                            a                                      1                    /                    2                                                  ⁢                                  o                  2                                ⁢                                  a                                      H                    2                                    2                                                            a                                                      H                    2                                    ⁢                  O                                                                                        [                  Formula          ⁢                                          ⁢          2                ]            
(In Formula 2, Eoo2 denotes a standard oxygen electrode potential.)
Then, it is the principle of fuel cells that the equilibrium electromotive force Eo2−EH2 given by Formula 1 and Formula 2 is made to be the electromotive force of the cell.
In mounting the solid polymer electrolyte fuel cells on automobiles, in reality, the solid polymer electrolyte fuel cells are used in the form of a stack (aggregated cells) comprising a combination of several tens to several hundreds of electrode assemblies, each of which is a basic unit of the fuel cell.
Incidentally, in case that a drastic output variation is generated in the fuel cells by drastically accelerating the vehicle, which incorporates a stack comprising solid polymer electrolyte fuel cells, a large amount of current needs to be supplied from the cells by increasing the supply amount of hydrogen as fuel on the fuel electrode side according to the output variation so generated.
However, readiness in controlling the supply lacks, since hydrogen supplied as fuel is in a gaseous state. The supply amount of hydrogen accordance with the increase of current does not increase in real time to follow the variation in output. The supply amount of the hydrogen increases later than a timing when the output variation occurs for supply.
Although the delay in supplying hydrogen is in the order of several seconds, there occurs a shortage of hydrogen as fuel, since the ionization reaction expressed by the reaction formula (1) is promoted at the fuel electrode immediately after the variation in output occurs in order to match the increase of current in association with the variation in output so that hydrogen is consumed more than before the variation in output occurs. Namely, it follows that the fuel cell lacks fuel.
Nonetheless, in order to maintain a large amount of current required by the variation in output, electrons are supplied from the fuel electrode side to compensate for the decrease in the supply amount of electrons by a reaction, which is expressed by the reaction formula 1 in conjunction with the shortage of hydrogen.
It is considered that the reaction then occurring at the fuel electrode is expressed by the following reaction formulae (3) and (4).2H2O→O2+4H++4e−  (3)C+2H2O→CO2+4H++4e−  (4)
Then, in conjunction with the reactions above the equilibrium potential of the fuel electrode corresponds to EH2 expressed by Formula 1 corresponding to the reaction formula (1), Eo2 expressed by Formula 2 corresponding to the reaction formula (3) and the reaction formula (4).
                              E                      CO            2                          =                              E                          CO              2                                +                                    RT                              2                ⁢                F                                      ⁢            ln            ⁢                                                            a                                      1                    /                    2                                                  ⁢                                  co                  1                                ⁢                                  a                                      H                    +                                    1                                                                              a                                      1                    /                    2                                                  ⁢                                  ca                                                            H                      2                                        ⁢                    O                                                                                                          [                  Formula          ⁢                                          ⁢          3                ]            
(In Formula 3, Eoo2 denotes a standard carbon dioxide electrode potential.)
ECo2 expressed by Formula 3 corresponds to a sum of multiplications of the respective reaction products by constants according to the molar rates thereof, or kEH2+mEo2+nECo2 (k, m, n denotes constants). Then, in this case, the equilibrium electromotive force of the fuel cell is expressed by Eo2−(kEH2+mEo2+nECo2), and in the event that the variation in output is large, Eo2<kEH2+mEo2+nECo2 and a reverse voltage is generated in the fuel cell. Then, the reverse voltage condition lasts in the order of several seconds until the delayed supply of fuel hydrogen is dissolved as described above.
Then, there is caused a problem that as this occurs, on the fuel electrode side carbon used as a catalyst carrier in a catalyst layer constituting the fuel electrode is corroded due to the reaction expressed by the reaction formula (4) to thereby deteriorate the performance of the fuel electrode. Then, this deterioration in the performance of the fuel electrode lowers the generating performance of the fuel cell.
With a view to preventing the occurrence of the problem, conventionally, WO 01/15247 discloses a fuel cell in which a catalyst for promoting the electrolysis of water is mixed into a catalyst layer of a fuel electrode.
In the reference, the corrosion of carbon that carries the catalyst is prevented by enhancing the reaction expressed by the reaction formula (3) and suppressing the reaction expressed by the formula (4) which progresses in parallel with the reaction formula (3).
In the above conventional fuel cell, however, there may be caused a problem that maintaining the moisture retention of the fuel electrode becomes difficult due to the shortage of water happening in conjunction with the electrolysis of water expressed by the reaction formula (3). With a view to preventing the occurrence of the problem, in a fuel cell disclosed in WO 01/15249, PTFE resin or graphite is added to a substrate layer or a catalyst layer so that the water concentration of a fuel electrode is increased.
The corrosion of carbon carrying the catalyst is prevented by enhancing the reaction expressed by the reaction formula (3) while maintaining the moisture retention of the fuel electrode and suppressing the reaction expressed by the reaction formula (4) which progresses in parallel with the reaction of the formula (3).
In addition, in a fuel cell disclosed in WO 01/15255, a catalyst for promoting the electrolysis of water is mixed into a catalyst layer of a fuel electrode. Further, PTFE resin or carbon in the form of graphite is added to a substrate layer or the catalyst layer of the fuel electrode, whereby the reaction expressed by the reaction formula (3) is enhanced while controlling the water concentration of the fuel electrode so as to maintain the moisture retention thereof, whereas the reaction expressed by the reaction formula (4) which progresses in parallel with the reaction of the reaction formula (3) is restrained the corrosion of carbon which carries the catalyst is thereby prevented.
Furthermore, in a fuel cell of WO 01/15254, the deterioration of a material forming a fuel electrode that would occur in association with the reaction expressed by the reaction formula (4) or the like is restrained by increasing the catalyst carrying rate of a catalyst layer or improving the resistance to corrosion of a catalyst carrier.
However, the fuel cells disclosed in the above publications are such as to restrain the corrosion of carbon, and the resistance to reverse voltage cannot be sufficiently satisfied by them.
Incidentally, in the event that the amount of oxygen produced by the reaction expressed by the reaction formula (3) increases, there may be caused by oxygen so produced various problems in a fuel electrode or an electrolyte membrane which constitutes an electrode assembly.
The corrosion of the carbon occurs by the chemical combination of oxygen so produced, and carbon carrying the catalyst, so that resistance of a whole fuel cell is increased.
A reaction occurring then is expressed by the following reaction formula (5).O2+C→CO2  (5)
In addition, in many cases, a platinum-alloy catalyst such as using a platinum-ruthenium alloy is used as the material of a catalyst for a fuel cell in order to prevent the CO-poisoning of platinum (Pt), and there may be caused a risk that oxygen produced by the reaction expressed by the reaction formula (3) decomposes the alloy catalyst to deteriorate the catalyzing performance thereof.
The decomposition of the catalyst occurring then is expressed by the following reaction formula (6).O2+Pt−Ru→Pt+RuO2  (6)
Platinum (Pt) decomposed by the decomposition reaction expressed by the reaction formula (6) is eventually poisoned by CO.
Then, oxygen produced by the reaction of the reaction formula (3) burns to combine with hydrogen surrounding the fuel electrode, thereby causing a risk that the catalyst layer or the electrolyte membrane is deteriorated or damaged.
In particular, since the conventional ion exchanging resin of fluorine plastic (Nafion by DuPont, Flemion by Asahi Glass or the like) is a high-cost fluorine containing resin, hydrocarbon polymer (polyether ketone) is sometimes used in electrolyte membranes used in fuel cells.
In a case where such hydrocarbon polymer is used, there may occur a risk that the skeleton of the hydrocarbon polymer is broken by oxygen produced by the reaction expressed by the reaction formula (3), whereby the electrolyte membrane is damaged, leading to a risk that the performance of the fuel cell cannot be exhibited.